Fig. 5

Fig. 5 Schematic representation of the role of the LiaFSR three-component regulatory system in fluoroquinolone susceptibility. Hypothetical model of the LiaFSR system modeled with AlphaFold.

A In the absence of a stimulus, LiaFSR remains an intact complex. LiaS exhibits phosphatase activity, ensuring LiaR remains unphosphorylated and the regulon remains inactive. LiaF senses cell envelope stress, resulting in conformational changes in LiaS. B Upon cell membrane disruption, for example by bacitracin, the complex dissociates. LiaS no longer dephosphorylates LiaR, allowing LiaR to become phosphorylated, dimerize and bind to DNA to activate transcription of the genes within the LiaR regulon. C In the absence of LiaS (∆liaS), there is constitutive phosphorylation of LiaR, likely due to the metabolite acetyl phosphate present in the cell. The LiaR regulon is highly upregulated, and the overexpression of these genes, specifically heat-shock genes and spxA2, result in increased sensitivity to ciprofloxacin and levofloxacin.